The invention relates to a brake booster for an anti-wheel locking motor vehicle brake system, which is activated by a brake pedal, through a travel limiting spring. The brake booster is provided with a control valve that monitors both the connection between a pressure source and a booster cylinder including a booster piston associated with a brake circuit, and the connection of the booster cylinder with a discharge reservoir. The booster is integrated into a hydraulic dual circuit brake system, each of whose brake circuits is integrated or self-contained.
There are basically three categories of brake force boosters for dual circuit brakes presently known: (1) Brake force boosters with two open brake circuits; (2) brake force boosters with one closed and one open brake circuit; and (3) brake force boosters with two closed brake circuits. The first category includes exterior-force brake systems in which a shift mechanism is used to keep one or two self-contained brake circuits available should the exterior force fail. Such exterior force brake systems have the disadvantage that if a malfunction in the energy supply occurs and the hydraulic fluid reservoir is exhausted, no further braking effect can be produced.
The above-mentioned disadvantage is avoided by brake systems provided with so-called "emergency pistons" or with brake circuit shift devices which guarantee that, if the energy supply fails, an additional brake pressure can be produced via a brake circuit joined thereto.
Such brake circuits have the great advantage, when used in connection with an anti-wheel locking mechanism, because anti-wheel locking pressure modulation involves the lowest costs of the three system types. Magnetic valves or a multiple valve are required for pressure modulation and adjustment.
The brake systems of the second category provide various methods for a solution. The disadvantage of this type of system is that, if the auxiliary energy fails, the braking effect in one of the brake circuits is also lost. In addition, the same disadvantage mentioned in respect to the first category is present also, that if there is a leak in one brake circuit, the hydraulic energy supply fails.
The systems of the third category are the so-called "average pressure boosters". These systems provide that the hydraulic energy supply originates from the hydraulic circuit of the servo control (circulating principle). The disadvantage in these systems is that of a relatively high construction expense. In addition, this system type is not suitable for use in the same circuit with an anti-wheel locking protection device.